Silicones are useful in a variety of applications by virtue of their unique combination of properties, including high thermal stability, good moisture resistance, excellent flexibility, high ionic purity, low alpha particle emissions, and good adhesion to various substrates. For example, silicones are widely used in the automotive, electronic, construction, appliance, and aerospace industries.
Silicone compositions comprising an organopolysiloxane resin, organohydrogen-silane, and hydrosilylation catalyst are known in the art. The organopolysiloxane resin is typically a copolymer containing T and/or Q siloxane units in combination with M and/or D units. However, such compositions cure to form silicone products having relatively low fracture toughness, rendering them unsatisfactory for certain applications, such as fiber-reinforced composites, interlayer dielectrics, and abrasion resistant coatings.
Various approaches to improving the fracture toughness of cured silicone resins are reported in the literature. For example, U.S. Pat. No. 6,310,146 to Katsoulis et al. discloses a hydrosilylation reaction curable composition comprising a silsesquioxane copolymer, a silyl-terminated hydrocarbon, and a hydrosilylation reaction catalyst. The '146 patent also teaches that the cured silsesquioxane resin has improved strength and toughness without significant loss of modulus.
U.S. Pat. No. 6,509,423 to Zhu discloses a silicone resin composition comprising (A) 100 parts by weight of an organopolysiloxane resin containing an average of greater than two alkenyl groups per molecule and having less than 1.5 mole % percent of silicon-bonded hydroxy groups, (B) an organohydrogensilane in an amount sufficient to cure the composition, (C) an effective amount of an inorganic filler, and (D) a catalytic amount of a hydrosilylation catalyst. The '423 patent teaches that the silicone composition cures to form a silicone product having a low coefficient of thermal expansion and superior fracture toughness.
U.S. Pat. No. 6,689,859 to Li et. al. discloses a hydrosilylation reaction curable composition comprising a) a silsesquioxane polymer, (b) a mixture of silane and/or siloxane cross-linking compounds and c) a hydrosilylation reaction catalyst. The '859 patent teaches that the curable composition is cured to form a cured resin having high fracture toughness and strength without the loss of elastic modulus.
U.S. Pat. No. 6,646,039 to Li et. al. discloses a hydrosilylation reaction curable composition comprising a) a silsesquioxane polymer, b) a cross-linking compound, c) a hydrosilylation reaction catalyst, and d) colloidal silica having a surface coating formed thereon. The '039 patent teaches that the curable composition is cured to form a cured resin having high fracture toughness and strength without the loss of elastic modulus and glass transition temperature.
Although the preceding references disclose silicone compositions that cure to form silicone products having high fracture toughness, the compositions are unsatisfactory for certain applications, for example, resin transfer molding, which require lower viscosity and higher flow. Although organic solvents can be used to reduce the viscosity of the silicone compositions, solvents also increase the VOC (volatile organic compound) content of the compositions. Moreover, compositions containing organic solvents cannot be satisfactorily cured in closed systems, such as molds.
Consequently, there is a need for a hydrosilylation addition-curable silicone composition having lower viscosity that cures to form a silicone product having superior fracture toughness.